Method of producing chlorine dioxide using sodium chlorate and a water-retaining substance impregnated in zeolite or in aqueous solution

ABSTRACT

A method for producing chlorine dioxide by activating zeolite crystals (which have been impregnated with metal chlorite such as sodium chlorite, and optionally also a water-retaining substance such as magnesium sulfate, potassium chloride, potassium hydroxide, or calcium chloride) with excess protons, or activating an aqueous solution of metal chlorite and such a water-retaining substance with excess protons. Proton generating species useful for the activation are acids such as acetic, phosphoric, and citric acid, and metal salts such as ferric chloride, ferric sulfate, ZnSO4, ZnCl2, CoSO4, CoCl2, MnSO4, MnCl2, CuSO4, CuCl2, and MgSO4. The activation can be performed by causing fluid to flow through a bed of zeolite crystals impregnated with calcium chloride (or other water-retaining substance) and sodium chlorite, and a bed of zeolite crystals impregnated with a proton generating species. The two beds can be physically mixed together or the fluid can flow sequentially through separate beds. The activation can also be performed by immersing impregnated zeolite crystals in (or spraying them with) acid or another proton generating species. To produce chlorine dioxide using a sodium chlorite-containing aqueous solution, the solution can be mixed or otherwise combined with acid. Other aspects of the invention are impregnated zeolite crystals (or other carriers) which are useful for producing chlorine dioxide and are stable until activated with protons. Presence in a sufficient amount of a water-retaining substance in the unactivated material reduces the rate of chlorine dioxide outgassing to no more than a negligible amount prior to activation.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a division of U.S. patent application Ser. No.09/274,455, filed Mar. 22, 1999, now U.S. Pat. No. 6,174,508, which is acontinuation-in-part application of U.S. application Ser. No.08/798,873, filed Feb. 11, 1997, which issued as U.S. Pat. No. 5,885,543on Mar. 23, 1999.

FIELD OF THE INVENTION

The invention relates to methods for producing chlorine dioxide, and tosubstances used in performing such methods. Each method produceschlorine dioxide by activating zeolite crystals (previously impregnatedwith a mixture of sodium chlorite and a water-retaining substance suchas calcium chloride) with protons (from an acid or other protongenerating species), or by activating an aqueous solution of awater-retaining substance (such as calcium chloride) and sodium chloritewith protons (from an acid or other proton generating species).

BACKGROUND OF THE INVENTION

Zeolites are hydrated metal aluminosilicate compounds with well-defined(tetrahedral) crystalline structures. Because zeolite crystals (bothnatural and synthetic) have a porous structure with connected channelsextending through them, they have been employed as molecular sieves forselectively absorbing molecules on the basis of size, shape, andpolarity.

Volumes packed with zeolite crystals (for example, small zeolitecrystals chosen to have size in the range from 0.2 mm to one quarterinch) have been employed in air (or other gas) and water filtrationsystems to selectively absorb contaminants from a flowing stream ofwater or gas.

U.S. Pat. No. 5,567,405, issued Oct. 22, 1996 (based on U.S. applicationSer. No. 08/445,025, filed May 19, 1995), and U.S. Pat. No. 5,573,743,issued Nov. 12, 1996 (based on U.S. application Ser. No. 08/445,076),teach methods for producing zeolite crystals impregnated with one ormore of sodium chlorite, acetic acid, phosphoric acid, and citric acid,and methods for producing chlorine dioxide by moving a fluid (e.g., airor water) relative to a bed of zeolite crystals impregnated with sodiumchlorite, and moving the fluid relative to another bed of zeolitecrystals impregnated with one of the following: phosphoric acid, aceticacid and citric acid. The two beds can be physically mixed together, orthe fluid can flow sequentially through distinct first and second beds.These references also teach a method for filtering a fluid by producingchlorine dioxide in the fluid (in the manner described in thisparagraph) and then absorbing the chlorine dioxide from the fluid.

U.S. patent application Ser. No. 08/704,086, filed Aug. 28, 1996,teaches a variation on the chlorine dioxide production method of U.S.Pat. No. 5,567,405, which includes the steps of moving a fluid through afirst bed of impregnated zeolite crystals (impregnated with at least oneof phosphoric acid, acetic acid, and citric acid) and then moving thefluid through a second bed of impregnated zeolite crystals (impregnatedwith sodium chlorite).

Chlorine dioxide (ClO₂) is useful for killing biological contaminants(such as microorganisms, mold, fungi, yeast and bacteria) and foroxidizing volatile organic chemicals which can contaminate fluid.

It is known to produce chlorine dioxide by: activating a metal chloritesolution by adding an acid thereto, activating a powdered composition(or other dry composition) by adding water thereto, or preparing anactivated dry composition which releases chlorine dioxide over time.U.S. Pat. No. 4,547,381 (issued Oct. 15, 1985) and U.S. Pat. No.4,689,169 (issued Aug. 25, 1987) mention these three techniques forproducing chlorine dioxide, and disclose in some detail one type of suchan activated dry composition. They teach that this activated drycomposition is a mixture of a “dry inert diluent,” a metal chlorite, anda dry agent capable of reacting with the metal chlorite in a dry stateto produce chlorine dioxide. The metal chlorite can be sodium chlorite,and the dry agent can be a “dry acid” such as granular citric acid. Theinert diluent can be diatomaceous earth, sodium chloride, sodiumsilicate, disodium sulfate, or magnesium chloride, or a combination oftwo or more thereof. The mixture releases chlorine dioxide over timeuntil the rate of chlorine dioxide release becomes low, and the patentsteach that the mixture can then be agitated for “renewed generation” ofchlorine dioxide.

However, an activated composition (such as that described in U.S. Pat.Nos. 4,547,381 and 4,689,169) is subject to undesirable storage andshipping losses, due to outgassing of chlorine dioxide before the timeof intended use of the composition.

Similarly, the inventor has found that sodium chlorite-impregnatedzeolite crystals (of the type described in referenced U.S. Pat. Nos.5,567,405 and 5,573,743) are also subject to undesirable storage andshipping losses, due to outgassing of chlorine dioxide therefrom beforethe time of their intended use (e.g., before fluid is caused to flowthrough both a bed of the sodium chlorite-impregnated zeolite crystalsand a bed of acid-impregnated zeolite crystals). Also, efforts toactivate sodium chlorite-impregnated zeolite crystals (of the typedescribed in referenced U.S. Pat. Nos. 5,567,405 and 5,573,743) withacid at low temperature (below 40 degrees Fahrenheit) may fail in thesense that they will not result in release of sufficient amounts ofchlorine dioxide.

There are also disadvantages to use of conventional metal chloritesolutions (of the type mentioned in U.S. Pat. Nos. 4,547,381 and4,689,169) to produce chlorine dioxide. For example, when such aconventional solution is activated (by adding an acid thereto) torelease chlorine dioxide gas, it is difficult or impossible to controlthe rate of release of the chlorine dioxide gas. Overproduction ofchlorine dioxide often results.

There are also disadvantages to use of conventional powdered or drycompositions of the type activated by adding water thereto to releasechlorine dioxide (as mentioned, for example, in U.S. Pat. Nos. 4,547,381and 4,689,169). Masschelein, in the book Chlorine Dioxide—Chemistry andEnvironmental Impact of Oxychlorine Compounds (published 1979 by AnnArbor Science Publishers Inc., Ann Arbor, Mich.) at page 140, describesone such dry mixture comprising sodium chlorite, and a solid organicanhydride containing 2 to 20% of a desiccating product such as calciumchloride. When such a conventional dry composition is activated (byadding water thereto) to release chlorine dioxide gas, it is typicallydifficult or impossible to control the rate of release of the chlorinedioxide to achieve chlorine dioxide release rates useful for suchapplications as air or water filtration. Overproduction of chlorinedioxide often results.

Until the present invention, it was not known how to produce a stable,unactivated substance (either in liquid or dry form) which does notrelease significant amounts of chlorine dioxide until activated byexposure to an acid, and which releases chlorine dioxide at a useful(and controllable) rate when exposed to an acid (even at temperaturesbelow 40 degrees Fahrenheit). Nor had it been known to use such astable, unactivated substance to produce controlled release of chlorinedioxide for filtering air or water.

SUMMARY OF THE INVENTION

In some embodiments, the invention is a method for producing chlorinedioxide by activating zeolite crystals (previously impregnated withsodium chlorite and calcium chloride) with excess protons or activatingan aqueous solution of sodium chlorite and calcium chloride with excessprotons. Typically, the excess protons are produced by exposing thecrystals to an acid (or other proton generating species), or exposingthe aqueous solution of sodium chlorite and calcium chloride to an acid(or other proton generating species). Examples of proton generatingspecies useful for such activation are acids such as acetic acid,phosphoric acid, citric acid, HCl, propionic acid, and sulfuric acid,and metal salts such as ferric chloride, ferric sulfate, ZnSO₄, ZnCl₂,CoSO₄, CoCl₂, MnSO₄, MnCl₂, CuSO₄, CuCl₂, and MgSO₄. In some embodimentsthe proton generating species releases excess protons upon exposure tothe impregnated zeolite crystals themselves. In other embodiments theproton generating species must itself be activated to cause it releaseexcess protons, so that the protons can in turn activate the impregnatedzeolite crystals.

In accordance with the invention, the step of activating zeolitecrystals with excess protons can be performed by causing a fluid (e.g.,air, another oxygen-containing fluid, or water) to flow through a bed ofzeolite crystals impregnated with sodium chlorite and calcium chloride,and a bed of zeolite crystals impregnated with the proton generatingspecies. The two beds can be physically mixed together, or the fluid canbe caused to flow sequentially through distinct first and second beds(preferably first through the bed containing proton generatingspecies-impregnated zeolite, but alternatively first through the bedcontaining sodium chlorite and calcium chloride-impregnated zeolite).Alternatively, chlorine dioxide is produced using sodium chlorite andcalcium chloride-impregnated zeolite crystals by immersing theimpregnated zeolite in (or spraying the impregnated zeolite with) acidor another proton generating species, or otherwise exposing theimpregnated zeolite to acid (preferably aqueous acetic, phosphoric,citric acid, HCl, sulfuric acid, propionic acid, or another acid, with aconcentration of 0.025% to 0.5%, in the sense that the acid comprises0.025% to 0.5% by weight of the combined impregnated zeolite and acid)or another proton generating species.

To produce chlorine dioxide using the inventive aqueous solution ofsodium chlorite and calcium chloride, the solution can be mixed (orotherwise combined) with acid. The liquid mixture can then be sprayed orcoated on a surface (or the liquid mixture can be absorbed in a materialsuch as a sponge, pad, mat, or the like, or simply placed in areservoir, container, box, or the like) from which chlorine dioxide gascan escape at a desired rate.

In any of the embodiments, the rate of chlorine dioxide release(following activation) can be controlled in any of several ways,including by appropriately selecting the concentration and amount of theactivating acid (or other proton generating species), and usingimpregnated zeolite having appropriately selected weight ratios of metalchlorite (e.g., sodium chlorite) or sodium chlorate to zeolite, anddeliquescent or water absorbing and retaining substance (e.g., calciumchloride) to zeolite.

Other embodiments of the invention are substances useful for producingchlorine dioxide, which are stable until activated with excess protons(in the sense that they do not release chlorine dioxide gas insignificant amounts until so activated with protons). One suchembodiment is one or more zeolite crystals impregnated with sodiumchlorite and calcium chloride. Preferably, the zeolite crystals aresmall (each of size in a range from 0.2 mm to one quarter inch). Alsopreferably, the crystals comprise 1%-6% sodium chlorite, 0.5%-2% calciumchloride, 0%-20% (4%-8% in some preferred embodiments) water, and72%-98.5% (8%-94.5% in some preferred embodiments) zeolite by weight.Another such embodiment is an aqueous solution of sodium chlorite and atleast one chemical selected from the group consisting of magnesiumsulfate, potassium chloride, potassium hydroxide, and calcium chloride,preferably comprising 1%-6% of sodium chlorite, 0.5%-2% of said at leastone chemical, and 92%-98.5% of water (by weight). Preferably, thesolution comprises 1%-6% sodium chlorite, 0.5%-2% calcium chloride, and92%-98.5% water (by weight).

In variations on any embodiment of the invention, a water-retainingsubstance other than calcium chloride, such as magnesium sulfate(MgSO₄), potassium chloride, or potassium hydroxide, is substituted forcalcium chloride. For example, zeolite crystals impregnated with sodiumchlorite and magnesium sulfate are within the scope of the invention.For another example, a method of producing chlorine dioxide byactivating an aqueous solution of sodium chlorite and magnesium sulfatewith an acid is also within the scope of the invention.

In other variations on any embodiment of the invention, a metal chloriteother than sodium chlorite is substituted for sodium chlorite. Forexample, zeolite crystals impregnated with calcium chloride (ormagnesium sulfate) and a metal chlorite other than sodium chlorite arewithin the scope of the invention.

The inventor has found that the presence (in a sufficient amount) ofcalcium chloride (or another water-retaining substance such as magnesiumsulfate, potassium chloride, or potassium hydroxide) in the unactivatedcomposition of the invention reduces the rate of chlorine dioxideoutgassing to no more than a negligible amount at times prior toactivation of the composition with protons, and yet allows release ofchlorine dioxide at a desired rate following activation of thecomposition with protons. One of the reasons for chlorine dioxideoutgassing (prior to activation with excess protons) from zeoliteimpregnated with metal chlorite (but not impregnated with awater-retaining substance such as calcium chloride) is migration ofprotons in the aluminosilicates comprising the zeolite.

In other embodiments, the invention is a method for producing chlorinedioxide, including the steps of providing a zeolite crystal mixture,wherein the mixture comprises zeolite crystals impregnated with sodiumchlorate and zeolite crystals impregnated with an oxidizer, andactivating the mixture with excess protons, and a zeolite crystalmixture for use in performing such method. In other embodiments, theinvention is a method for producing chlorine dioxide, including thesteps of providing a zeolite crystal mixture, wherein the mixturecomprises zeolite crystals impregnated with a proton generating speciesand zeolite crystals impregnated with an oxidizer (or sodium chlorate),and causing the zeolite crystal mixture to come in contact with sodiumchlorate solution (or an oxidizer, where the mixture includes zeolitecrystals impregnated with sodium chlorate but not an oxidizer), and azeolite crystal mixture for use in performing such method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a zeolite crystal impregnated with ametal chlorite, and one of calcium chloride (or other deliquescent orwater absorbing and retaining substance), magnesium sulfate, potassiumchloride, and potassium hydroxide.

FIG. 2 is a cross-sectional view of a zeolite crystal impregnated with aproton generating species, or another substance or mixture of substancesin accordance with any of the embodiments of the invention.

FIG. 3 is a cross-sectional view of a mixture of impregnated zeolitecrystals (prepared in accordance with any embodiment of the invention)in a sealed container.

FIG. 4 is a cross-sectional view of a mixture of impregnated zeolitecrystals (prepared in accordance with the invention) surrounded by abarrier permeable to chlorine dioxide gas.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one class of embodiments, the invention is a process for impregnatingzeolite crystals with sodium chlorite and calcium chloride, and theproduct of such process. The zeolite crystals can have size (i.e.,largest dimension) equal (or approximately equal) to 0.125 inch, 0.25inch, 0.50 inch, or 0.75 inch, or size in the range from 0.2 mm toseveral millimeters, or size in the range from 0.2 mm to 0.25 inch, orthe zeolite crystals can have dimensions equal or substantially equal to0.25 inch×0.167 inch, 0.125 inch×0.10 inch, 0.25 inch×0.125 inch, 0.125inch×0.50 inch, or 0.50 inch×0.75 inch. The impregnation processproduces zeolite crystals uniformly impregnated (throughout the volumeof each crystal) with sodium chlorite and calcium chloride.

Preferred embodiments of this impregnation process employ, as inputmaterial, zeolite crystals whose moisture content has been reduced(substantially below an initial moisture content) to a low level(preferably about 5%). Such input material is preferably produced bymining zeolite, crushing the mined mineral into appropriately sizedzeolite crystals (having a natural moisture content substantially above5%), and then dehydrating the zeolite crystals until their moisturecontent is reduced to about 5%.

The dehydrated zeolite crystals are then immersed in (or sprayed with)an aqueous solution of sodium chlorite and calcium chloride at hightemperature (e.g., in the range from 120° F. to 190° F.), and theresulting slurry is thoroughly mixed. Then, the mixed slurry is airdried (or allowed to equilibrate) to the desired moisture level(typically 0-20% ) to produce impregnated zeolite crystals.Alternatively, the air drying step can be avoided by calculating theamount of aqueous impregnating material needed to achieve the desiredfinal moisture level (e.g., 15%) and adding this amount to thedehydrated zeolite at the time of impregnation.

Alternatively, the dehydrated zeolite crystals are immersed in (orsprayed with) an aqueous solution of sodium chlorite at high temperature(e.g., at least 190° F.), and the resulting slurry is thoroughly mixed.Then, the mixed slurry is air dried (or allowed to equilibrate) to thedesired moisture level to produce sodium chlorite-impregnated zeolitecrystals. Alternatively, one calculates the amount of aqueous sodiumchlorite needed to achieve the desired moisture level (e.g., a desiredlevel in the range 15%-20%) and adds this amount to the dehydratedzeolite at the time of impregnation. Then, the sodiumchlorite-impregnated zeolite crystals are immersed in (or sprayed with)an aqueous solution of calcium chloride at high temperature, and theresulting slurry is thoroughly mixed. Then, the mixed slurry is airdried (or allowed to equilibrate) to the desired final moisture level(typically 0-20%) to produce zeolite crystals impregnated with bothsodium chlorite and calcium chloride. Alternatively, air drying isavoided by calculating the amount of aqueous calcium chloride needed toachieve the desired final moisture level (e.g., a desired level in therange 15%-20%) and adding this amount to the sodium chlorite-impregnatedzeolite at the time of calcium chloride impregnation.

In variations of any of the described zeolite impregnation processes, awater-retaining substance such as magnesium sulfate (MgSO₄), potassiumchloride, or potassium hydroxide is substituted for calcium chloride asan impregnating agent.

In other variations on the described zeolite impregnation processes, ametal chlorite other than sodium chlorite is substituted for sodiumchlorite.

FIG. 1 represents one impregnated zeolite crystal, having channelsuniformly impregnated with mixture 2 of metal chlorite and awater-retaining substance. The water-retaining substance is preferablycalcium chloride, but alternatively is magnesium sulfate, potassiumchloride, potassium hydroxide, or another water-retaining substance. Themetal chlorite is preferably sodium chlorite.

The rate at which impregnated zeolite crystals of the type shown in FIG.1 (e.g., zeolite crystals uniformly impregnated sodium chlorite andcalcium chloride) react with in the presence of excess protons torelease chlorine dioxide can be controlled (reduced or increased to adesired level) by varying the relative amounts (by weight) of thedifferent impregnating agents therein.

In some embodiments of the invention, the proton generating species isin the form of zeolite crystals impregnated with one or more metal salts(preferably one or more of ferric chloride, ferric sulfate, ZnSO₄,ZnCl₂, CoSO₄, CoCl₂, MnSO₄, MnCl₂, CuSO₄, CuCl₂, and MgSO₄). Uponexposure of the metal salt or salts to water (either liquid water or amoisture-containing gas, e.g. atmospheric moisture), excess protons aregenerated.

In other embodiments of the invention, the proton generating species isactivated (other than by exposure to water) to release excess protons.The proton generating species is activated in such a manner thatimpregnated zeolite crystals are exposed to excess protons, therebyinitiating a reaction resulting in release of chlorine dioxide gas. Insome embodiments, the proton generating species releases excess protonsupon exposure to the impregnated zeolite crystals themselves (theimpregnated zeolite crystals together with the proton generating speciesdoes not need to be further activated to cause the proton generatingspecies to release excess protons).

In some embodiments of the invention, the proton generating species isin the form of zeolite crystals impregnated with at lease one acid(preferably one or more of acetic acid, citric acid, phosphoric acid,HCl, propionic acid, and sulfuric acid). Above-referenced U.S. Pat. Nos.5,567,405 and 5,573,743 disclose methods for producing suchacid-impregnated zeolite crystals. Briefly, the acid impregnation stepis preferably performed by immersing dehydrated zeolite crystals in (orspraying zeolite crystals with) an aqueous solution of one or more ofthe acids at high temperature, thoroughly mixing the resulting slurry,and finally air drying (or allowing the slurry to equilibrate) to thedesired moisture level (e.g. 0%-20%) the mixed slurry to produce theacid-impregnated zeolite crystals. FIG. 2 represents one suchimpregnated crystal, having channels uniformly impregnated (throughoutthe volume of the crystal) with chemical 10, where chemical 10 is aceticacid, citric acid, phosphoric acid, HCl, propionic acid, or sulfuricacid (or another acid), or a mixture of two or more of such acids, oranother proton generating species, or another substance or mixture ofsubstances in accordance with any of the embodiments of the invention.

In general, the acid or acids employed (for activation of impregnatedzeolite crystals) in accordance with the invention can be impregnated inzeolite crystals, mixed in an aqueous solution (including a metalchlorite and a water-retaining substance such as calcium chloride), oradded to (e.g., sprayed on) zeolite crystals that have been impregnatedwith a metal chlorite and a water-retaining substance such as calciumchloride.

With reference again to zeolite crystals that have been impregnated inaccordance with the invention with sodium chlorite and calcium chloride,such crystals are preferably small (each of size in a range from 0.2 mmto one quarter inch), and each crystal preferably comprises 1%-6% sodiumchlorite, 0.5%-2% calcium chloride, 0%-20% (4%-8% in some preferredembodiments) water, and 72%-98.5% (84%-94.5% in some preferredembodiments) zeolite by weight. The inventor has found that the presenceof calcium chloride (preferably in the preferred amount noted above) inthe unactivated impregnated zeolite reduces the rate of chlorine dioxideoutgassing to no more than a negligible amount until activation of thecomposition with acid, and yet allows release of chlorine dioxide at adesired rate following activation of the composition with acid. Thisfinding was unexpected, since mixing of sodium chlorite and calciumchloride with water would have been expected to produce chlorine dioxideas a result of the following sequence of reactions:

1. 2NaClO₂+2H₂O+CaCl₂→

2. Ca(OH)₂+2HCL+2NaClO₂→

3. HCL+NaClO₂+NaCl+ClO₂↑+protons.

Instead, the unexpected finding that the rate of chlorine dioxideoutgassing is substantially reduced is believed to result from thefollowing reaction:

1. 2NaClO₂+H₂O+CaCl₂→

2. H₂O+CaClO₂+2NaCl.

The products of this reaction, when activated with excess protons (e.g.,by exposure to acid or a metal salt) are converted to ClO₂↑ and othersubstances including excess protons. In another embodiment, when sodiumchlorite and ferric chloride react with water to produce chlorinedioxide gas, it is believed that the chlorine dioxide is produced as aresult of the following reaction:

3NaClO₂+3H₂O+FeCl₃→3ClO₂+(Fe) (OH)₃+3NaCl.

In another class of embodiments, the invention is an aqueous solution ofsodium chlorite and calcium chloride (or sodium chlorite and awater-retaining substance other than calcium chloride). This solutioncan be activated by exposure to protons (e.g., as a result of exposureto a proton generating species such as acetic acid, phosphoric acid,citric acid, or another acid) so as to release chlorine dioxide in acontrolled manner. Preferably, the solution comprises 1%-6% sodiumchlorite, 0.5%-2% calcium chloride, and 92%-98.5% water (by weight).Also preferably, the solution is activated by being mixed (or otherwisecombined) with aqueous acetic acid, phosphoric acid, citric acid, HCl,propionic acid, or sulfuric acid (or another acid) having aconcentration of 0.025% to 0.5% (by weight of the total mixture). Anexample of such an aqueous acid is produced by adding 1% (by weight ofthe total mixture) of a 3% aqueous acetic acid solution. The activatedliquid mixture (of aqueous sodium chlorite, calcium chloride, and acid)can then be sprayed or coated on a surface (or the liquid mixture can beabsorbed in a material such as a sponge, pad, mat, or the like, orsimply placed in a reservoir) from which chlorine dioxide gas can escapeat a desired rate.

The inventor has found that the presence of diluted calcium chloride orone of the alternative water-retaining substances (preferably in theabove-noted preferred amount of 0.5%-2% by weight) in the unactivatedsolution reduces the rate of chlorine dioxide outgassing to no more thana negligible amount at times prior to activation with an acid, and yetallows release of chlorine dioxide at a desired rate upon activation ofthe solution with acid. The unexpected finding that the rate of chlorinedioxide outgassing (prior to activation) is substantially reduced isbelieved to result from the following reaction in the unactivatedsolution:

1. 2NaClO₂+H₂O+CaCl₂→

2. H₂O+CaClO₂+2NaCl.

The products of this reaction, when activated with excess protons (e.g.,by addition of acid) are converted to ClO₂↑ and other substancesincluding excess protons.

The impregnated zeolite crystals of the invention (crystals impregnatedwith a metal chlorite and calcium chloride, or a metal chlorite and awater-retaining substance other than calcium chloride) are useful in aclass of methods for producing chlorine dioxide (ClO₂). Such methodswill be described in preferred embodiments in which calcium chloride isan impregnating agent and the metal chlorite is sodium chlorite, but itshould be understood that a water-retaining substance (such as magnesiumsulfate, potassium chloride, or potassium hydroxide) can be substitutedfor calcium chloride as an impregnating agent in the methods (and that ametal chlorite other than sodium chlorite can be substituted for sodiumchlorite).

In one chlorine dioxide-producing method in accordance with theinvention, a fluid (preferably a fluid containing oxygen, such as air orwater) is caused to move relative to a first bed (i.e., the fluid flowsthrough the first bed or the crystals comprising the first bed movethrough the fluid) of zeolite crystals impregnated with: (1) phosphoricacid (H₃PO₄), (2) acetic acid (CH₃COOH), (3) citric acid, (4) ferricchloride (FeCl₃), (5) ferric sulfate, or (6) another metal salt, such asZnSO₄, ZnCl₂, COSO₄, CoCl₂, MnSO₄, MnCl₂, CuSO₄, CuCl₂, or MgSO₄, or (7)an acid other than phosphoric, acetic, or citric acid, or another protongenerating species suitable for the particular application. Then, thefluid is caused to move relative to a second bed (i.e., the fluid flowsthrough the second bed, or the crystals comprising the second bed movethrough the fluid) of zeolite crystals which are impregnated with amixture of sodium chlorite (NaClO₂) and calcium chloride (CaCl₂). Uponmoving the fluid relative to the second bed, chlorine dioxide isreleased (due to contact with acid or other proton generating substancetransferred from the first bed). It is believed that the chlorinedioxide release occurs as a result of the following reaction, in thecase of a first bed impregnated with acetic acid:

CH₃COOH+CaClO₂+2NaCl+H₂O→ClO₂↑+Ca(CH₃COO)₂+excess protons,

where Ca(CH₃COO)₂ is calcium acetate.

As discussed above, CaClO₂, 2NaCl, and H₂O are believed to be present inthe channels of the zeolite crystals of the first bed as a result ofreaction of the impregnating agents NaClO₂ and CaCl₂, and H₂O, in thechannels of the zeolite crystals of the first bed.

In variations on this chlorine dioxide production method, the first bedcan include a mixture of phosphoric acid-impregnated zeolite crystalsand acetic acid-impregnated zeolite crystals, or a mixture of phosphoricacid-impregnated zeolite crystals and citric acid-impregnated zeolitecrystals, or a mixture of acetic acid-impregnated zeolite crystals andcitric acid-impregnated zeolite crystals, or a mixture of all threetypes of acid-impregnated zeolite crystals, or a mixture of zeolitecrystals impregnated with one of these three types of acids and zeolitecrystals impregnated with another acid (e.g., HCl or H₂SO₄).

It is preferable for the fluid to flow through a first bed of protongenerating species-impregnated (e.g., acid-impregnated) zeolite crystalsbefore the fluid flows through a second bed containing sodium chloriteand calcium chloride-impregnated zeolite crystals, since this sequencewill result in protons entering the fluid (due to interaction of thefluid with the acid or other proton generating species in the crystalsof the first bed), and since the presence of the hydrogen ions in thefluid will enhance chlorine dioxide production when the hydrogenion-containing fluid interacts with the impregnating chemical in thecrystals of the second bed.

While the foregoing process for producing ClO₂ has been described withreference to two distinct (first and second) beds of impregnated zeolitecrystals, a single bed containing a mixture of crystals can be used aslong as the mixed bed contains both impregnated zeolite crystals fromthe first bed described above and zeolite crystals from the second beddescribed above. As an example, a mixed bed of zeolite crystals (zeolitecrystals impregnated with sodium chlorite and calcium chloride mixedwith zeolite crystals impregnated with phosphoric acid) can be used toproduce chlorine dioxide (e.g., by flowing a fluid through the mixedbed).

Alternatively, chlorine dioxide can be produced by adding aqueous acidto a bed of zeolite crystals impregnated with a metal chlorite (e.g.,sodium chlorite) and a water-retaining substance (e.g., calciumchloride). In these alternative embodiments, the rate of release ofchlorine dioxide can be controlled by varying the amount andconcentration of the added acid.

In alternative embodiments, chlorine dioxide is produced with a reversedsequence of distinct first and second beds, as follows. A fluid(preferably a fluid containing oxygen such as air) is caused to moverelative to a first bed (i.e., the fluid flows through the first bed, orthe crystals comprising the first bed move through the fluid). The firstbed comprises zeolite crystals impregnated with sodium chlorite (NaClO₂)and calcium chloride (CaCl₂). Then, the fluid is caused to move relativeto a second bed (i.e., the fluid flows through the second bed, or thecrystals comprising the second bed move through the fluid) of zeolitecrystals which are impregnated with: (1) phosphoric acid, (2) aceticacid, (3) citric acid, (4) ferric chloride (FeCl₃), (5) ferric sulfate,or (6) another metal salt, such as ZnSO₄, ZnCl₂, CoSO₄, CoCl₂, MnSO₄,MnCl₂, CuSO₄, CuCl₂, or MgSO₄, or (7) an acid other than phosphoric,acetic, or citric acid, or another proton generating species suitablefor the particular application. Upon moving the fluid relative to thesecond bed, chlorine dioxide is released. As with other embodimentsdescribed herein, a water-retaining substance such as magnesium sulfate,potassium chloride, or potassium hydroxide can be substituted for thecalcium chloride (and a metal chlorite other than sodium chlorite can besubstituted for the sodium chlorite).

In another class of embodiments of the inventive chlorine dioxideproduction method, zeolite crystals impregnated with sodium chlorite andcalcium chloride are activated by being immersed in (or sprayed with)aqueous acetic acid, phosphoric acid, citric acid, HCl, or sulfuricacid, with a concentration of 0.025% to 0.5% (in the sense that the acidcomprises 0.025% to 0.5% by weight of the combined impregnated zeoliteand acid). Alternatively, another acid or other proton generatingspecies suitable for the particular application can be used to immerse(or spray) the impregnated zeolite crystals as a substitute for theaqueous acetic acid, phosphoric acid, citric acid, HCl, or sulfuricacid.

While chlorine dioxide can kill. microorganisms in the fluid undergoingtreatment, and can oxidize volatile organic chemicals which contaminatethe fluid undergoing treatment (as described above), chlorine dioxideitself is a contaminant. Therefore, it is sometimes desirable to removechlorine dioxide from the fluid stream after the chlorine dioxide hasperformed purification (including biological purification) of the fluidstream. Another aspect of the present invention relates to a multi-stepfiltration process in which chlorine dioxide is first produced in afluid stream and, subsequently, removed from the fluid stream.

First, a fluid is caused to be moved relative to a primary bed (orprimary beds) of zeolite crystals (i.e., the described first and secondbeds, or the described mixed bed) which will cause chlorine dioxide tobe released, as described above. While the fluid moves relative to(e.g., while the fluid flows through) the primary bed or beds, chlorinedioxide is released (generally as a gas). The released chlorine dioxidewill kill biological contaminants in the fluid and will oxidize volatileorganic chemicals which contaminate the fluid.

Then, the fluid is moved relative to a secondary bed (filter) of zeolitecrystals impregnated with one of the following: (1) potassium hydroxide(KOH), (2) sodium sulfite, (3) sodium bisulfite, and (4) ferrous sulfate(i.e., the fluid flows through the secondary bed or the crystalscomprising the secondary bed move through the fluid). The zeolitecrystals of the secondary bed react with the chlorine dioxide to removethe chlorine dioxide from the fluid. Some chemical equations describingpossible reactions in the secondary bed follow.

For a secondary bed of potassium hydroxide:

2KOH+2ClO₂→KClO₂+KCLO₄+H₂O

For a secondary bed of sodium sulfite:

ClO₂+Na₂SO₃→H₂O+S₂O₅+H⁺ ₃ClO₃

Note that H⁺⁺ from the acids involved in the previous reactions cancelthis reaction.

For a secondary bed of sodium bisulfite:

ClO₂+Na₂SO₅→H₂O+S₂O₅+H⁺ ₃ClO₃

Note that H⁺⁺ from the acids involved in the previous reactions cancelthis reaction.

The preferred composition of the secondary bed is ferrous sulfate. For asecondary bed of ferrous sulfate:

ClO₂+Fe⁺⁺SO₄→Fe⁺⁺⁺+Cl⁻+ClO₂ ⁻→FeCl₃ (primarily).

When using a secondary bed of ferrous sulfate a color change, from whiteto brown, takes place. This is an indicator of neutralization.

Various modifications and variations of the described methods andcompositions of the invention will be apparent to those skilled in theart without departing from the scope and spirit of the invention.

For example, a carrier other than zeolite crystals or water (e.g.,pumice, diatomaceous earth, bentonite, or clay) can be used to carry themetal chlorite and water-retaining substance (and optionally also theacid or other proton generating species) of the invention.

We next describe other classes of embodiments of the invention.

In one such class of embodiments, the invention is a method forproducing chlorine dioxide by activating a mixture comprising zeolitecrystals impregnated with sulfuric acid (or other proton generatingspecies), zeolite crystals impregnated with sodium chlorate, zeolitecrystals impregnated with an oxidizer, and optionally also zeolitecrystals impregnated with calcium chloride (or another deliquescent orwater absorbing and retaining substance). The oxidizer is a substance(or mixture of substances) which reacts (in the presence of waterabsorbed by the deliquescent or water absorbing substance) to releaseanother substance which in turn reacts with one or more of the otherimpregnating substances to produce chlorine dioxide. In some of theseembodiments, the oxidizer is ferric chloride or ferric sulfate, or amixture of ferric chloride and ferric sulfate. Although each of theseiron salts desirably absorbs water in addition to functioning as anoxidizer, ferric chloride is preferably used (rather than ferricsulfate, or a mixture of ferric chloride and ferric sulfate) since it isthe most reactive during performance of the inventive method. Wheresodium chlorate, ferric chloride, and sulfuric acid are present uponactivation, the chlorine dioxide-producing reaction is believed to be:

2Na(ClO₃)+FeCl₃+H₂SO₄+3H₂O→Na₂SO₄+Fe(OH)₃+3HCl+2H₂O+2ClO₂↑.

In any of this class of embodiments, the activation can be accomplishedby exposing the mixture to a moisture-containing gas (e.g., airincluding water vapor), or (to achieve a higher chlorine dioxideproduction rate) causing water or a moisture-containing gas to flowthrough the mixture (or by causing the mixture to move through the gas).When activation is accomplished using a flowing fluid, all theimpregnated zeolite crystals can be physically mixed together in asingle bed, or the gas can be caused to flow sequentially throughdistinct beds of subsets of the zeolite crystals.

In other embodiments, chlorine dioxide is produced-by activating (withexcess protons) a zeolite crystal mixture comprising zeolite crystalsimpregnated with sodium chlorate, zeolite crystals impregnated with atleast one oxidizer (e.g., hydrogen peroxide, ferric chloride, orperacetic acid), and optionally also zeolite crystals impregnated withcalcium chloride. Preferably, the mixture comprises equal (orsubstantially equal) amounts of crystals impregnated with each of thefour species. The activation can be accomplished by exposing the mixtureto a moisture-containing gas (e.g., air including water vapor), or (toincrease the chlorine dioxide production rate) causing water or amoisture-containing gas (e.g., air including water vapor) to flowthrough the mixture. When activation is accomplished using flowingfluid, all impregnated zeolite crystals can be physically mixed togetherin a single bed, or the fluid can be caused to flow sequentially throughdistinct beds of subsets of the zeolite crystals. In embodiments inwhich the impregnated crystal mixture comprises distinct beds ofdifferent subsets of zeolite crystals (e.g., a bed of crystalsimpregnated with a first substance, and another bed of crystalsimpregnated with a second substance mixed with crystals impregnated witha third substance), the fluid must flow sequentially through the beds toaccomplish activation.

Where sodium chlorate, hydrogen peroxide, and sulfuric acid are presentupon activation, the chlorine dioxide-producing reaction is believed tobe

2Na(ClO₃)+H₂O₂+H₂SO₄→2ClO₂↑+Na₂SO₄+H₂O.

In variations on the embodiments described in the previous paragraph,zeolite crystals impregnated with sodium metabisulfite (or sodiumbisulfite) are used rather than zeolite crystals impregnated with atleast one of hydrogen peroxide, ferric chloride, and peracetic acid.Activation of the mixture by absorbed water (water absorbed by thedeliquescent or water absorbing and retaining substance) causes areaction of the sodium metabisulfite (or sodium bisulfite) whichreleases sulfur dioxide (SO₂). The SO₂ then reacts to form chlorinedioxide gas. Each of sodium metabisulfite and sodium bisulfite isconsidered an “oxidizer” in the sense that the latter expression is usedherein, since each reacts (in the presence of the absorbed water) torelease sulfur dioxide which in turn reacts with one or more of theother impregnating substances to produce chlorine dioxide.

In other embodiments, chlorine dioxide is produced by activating azeolite crystal mixture comprising zeolite crystals impregnated withsodium chlorate, zeolite crystals impregnated with sulfuric acid (oranother proton generating species, such as ferric chloride, ferricsulfate, ZnSO₄, ZnCl₂, CoSO₄, CoCl₂, MnSO₄, MnCl₂, CuSO₄, CuCl₂, andMgSO₄ or another metal salt), zeolite crystals impregnated with anoxidizer, and optionally also zeolite crystals impregnated with adeliquescent (or water absorbing and retaining substance). As in otherembodiments, activation of the zeolite crystal mixture can beaccomplished by exposing the mixture to a moisture-containing gas (e.g.,air including water vapor), or (in order to increase the chlorinedioxide production rate) by causing water or a moisture-containing gasto flow through the mixture (or causing the mixture to move through suchfluid). When activation is accomplished using flowing fluid, theimpregnated zeolite crystals can be physically mixed together in asingle bed or the fluid can be caused to flow sequentially throughdistinct beds of subsets of the zeolite crystals. In some of theseembodiments, the deliquescent (or water absorbing and retainingsubstance) is magnesium sulfate (MgSO₄), potassium chloride, ormagnesium chloride (MgCl).

In other embodiments, the invention is a method for producing chlorinedioxide by activating an impregnated zeolite crystal mixture with anoxidizer (such as ferric chloride solution (preferably) or liquidhydrogen peroxide or liquid peracetic acid). The zeolite crystal mixturecomprises zeolite crystals impregnated with sodium chlorate, zeolitecrystals impregnated with sulfuric acid (or another proton generatingspecies), and optionally also zeolite crystals impregnated with calciumchloride (or another deliquescent or water absorbing and retainingsubstance).

In other embodiments, the invention is a method for producing chlorinedioxide by activating an impregnated zeolite crystal mixture with excessprotons, such as by exposing it to liquid sulfuric acid (or anotherproton generating species). The zeolite mixture comprises zeolitecrystals impregnated with sodium chlorate, zeolite crystals impregnatedwith an oxidizer (e.g., at least one of hydrogen peroxide, ferricchloride, and peracetic acid), and optionally also zeolite crystalsimpregnated with calcium chloride (or another deliquescent or waterabsorbing and retaining substance).

In other embodiments, the invention is a method for producing chlorinedioxide by activating an impregnated zeolite crystal mixture with sodiumchlorate solution. The zeolite mixture comprises zeolite crystalsimpregnated with sulfuric acid (or is another proton generatingspecies), zeolite crystals impregnated with an oxidizer (e.g., at leastone of hydrogen peroxide, ferric chloride, and peracetic acid), andoptionally also zeolite crystals impregnated with calcium chloride (oranother deliquescent or water absorbing and retaining substance).

Also within the scope of the invention are methods in which mixtures oftwo or more of the above-described impregnated zeolite mixtures areactivated to produce chlorine dioxide. An example is a method forproducing chlorine dioxide by activating a zeolite crystal mixturecomprising zeolite crystals impregnated with sulfuric acid, zeolitecrystals impregnated with sodium chlorate, zeolite crystals impregnatedwith at least one of hydrogen peroxide, ferric chloride, and peraceticacid, zeolite crystals impregnated with calcium chloride, and zeolitecrystals impregnated with sodium metabisulfite (or sodium bisulfite).

In any embodiment of the invention, the rate of chlorine dioxide release(upon activation of the zeolite crystal mixture) can be controlled inany of several ways, including by appropriately selecting theconcentration and amount of an activating liquid (e.g., where theactivating liquid is sodium chlorate solution or liquid sulfuric acid),using impregnated zeolite crystals having an appropriately selectedweight ratio of one or more of the impregnating substances (e.g., ironsalt) to zeolite, and selecting an appropriate method for activating themixture. For example, activation by exposing the mixture tomoisture-containing gas typically results in a low chlorine dioxiderelease rate, and activation by flowing water or moisture-containing gasthrough the mixture typically results in a higher chlorine dioxiderelease rate. Prior to activation, the mixture of impregnated crystalsshould be as dry as possible. Preferably, the mixture (e.g., mixture ofimpregnated zeolite crystals 40 of FIG. 3) is sealed within a dry,air-tight capsule or other container (e.g., container 41 of FIG. 3), andthe container is unsealed to expose the mixture to the activating fluidshortly before activation.

In yet another class of embodiments, the invention is a mixture ofimpregnated zeolite crystals which can be activated (by any of theabove-described liquids or by water or moisture-containing gas inaccordance with the invention) to perform any embodiment of theinventive chlorine dioxide production method.

The chlorine dioxide produced in accordance with the invention can beused to kill microorganisms in the moisture-containing gas whichactivates the crystal mixture, and can oxidize volatile organicchemicals which contaminate the moisture-containing gas.

In another class of embodiments, the invention is composition of matterwhich carries sufficient amounts of chemicals to be capable of releasingchlorine dioxide to a target region upon activation by excess protons,said composition of matter comprising a quantity of impregnated zeolitecrystals including zeolite crystals impregnated with a metal chloriteand zeolite crystals impregnated with a proton generating species,wherein the quantity is capable of being activated by excess protons toproduce chlorine dioxide. Zeolite crystals 30 of FIG. 4 are such aquantity of impregnated zeolite crystals. Preferably, those of crystals30 which are impregnated with a metal chlorite are also impregnated withat least one of a deliquescent and a water absorbing and retainingsubstance (e.g., they are impregnated with calcium chloride) in aconcentration which limits (to a predetermined maximum level) the rateof release of chlorine dioxide to the target region in response toactivation of the quantity of crystals with excess protons. For example,zeolite crystals 30 can include zeolite crystals impregnated with metalchlorite, a water absorbing and retaining substance, and an acid (orother proton generating species). Such crystals will be activated byliquid water or atmospheric moisture (which will cause production ofexcess protons, which in turn will cause chlorine dioxide production).The relative concentrations of impregnating chemicals will control therate at which excess protons are produced (and thus the rate at whichchlorine dioxide is produced) in response to a specific amount of water.

For some applications, the quantity of crystals is separated by abarrier (e.g., barrier 50 of FIG. 4) from the target region. The barriershould be permeable to flow of an activating fluid (e.g., water) fromthe target region such that the activating fluid will interact with thequantity of impregnated zeolite crystals in a manner resulting inproduction of excess protons (which in turn results in production ofchlorine dioxide). The barrier should also be permeable to flow of thechlorine dioxide to the target region. Preferably, the permeability ofthe barrier is such that the quantity of zeolite crystals is exposed tono more than predetermined maximum flow rate of the activating fluid(e.g., the barrier is permeable to flow of the activating fluid from thetarget region at up to a predetermined maximum flow rate), so that thebarrier limits to a predetermined maximum level the rate of release ofchlorine dioxide to the target region.

Although the invention has been described in connection with specificpreferred embodiments, it should be understood that the invention asclaimed should not be unduly limited to such specific embodiments.

What is claimed is:
 1. A method for producing chlorine dioxide, including the steps of: (a) providing a zeolite crystal mixture which comprises zeolite crystals impregnated with sodium chlorate and zeolite crystals impregnated with an oxidizer; and (b) activating the zeolite crystal mixture with excess protons, thereby causing the zeolite crystal mixture to produce the chlorine dioxide.
 2. The method of claim 1, wherein step (b) includes the step of exposing the zeolite crystals impregnated with the sodium chlorate and the zeolite crystals impregnated with the oxidizer to a proton generating species.
 3. The method of claim 1, wherein the zeolite crystal mixture also comprises zeolite crystals impregnated with a proton generating species, and wherein step (b) includes the step of causing the zeolite crystal mixture to contact a moisture-containing fluid, whereby the zeolite crystal mixture is activated to produce the chlorine dioxide.
 4. The method of claim 3, wherein the zeolite crystal mixture is stored in a dry, air-tight container prior to step (b), and wherein step (b) includes the step of opening said container to expose the zeolite crystal mixture to the moisture-containing fluid.
 5. The method of claim 3, wherein step (b) includes the step of causing the moisture-containing fluid to flow through the zeolite crystal mixture, whereby the mixture is activated by water absorbed from the moisture-containing fluid and as a result said mixture produces the chlorine dioxide.
 6. The method of claim 3, wherein the proton generating species is sulfuric acid.
 7. The method of claim 1, wherein the zeolite crystal mixture also includes zeolite crystals impregnated with at least one of a deliquescent and a water absorbing and retaining substance.
 8. The method of claim 1, wherein the zeolite crystals impregnated with said oxidizer are zeolite crystals impregnated with at least one of hydrogen peroxide, ferric chloride, peracetic acid, sodium metabisulfite, and sodium bisulfite.
 9. The method of claim 1, wherein the zeolite crystal mixture, the zeolite crystal mixture also comprises zeolite crystals impregnated with calcium chloride, wherein the oxidizer is at least one of hydrogen peroxide, ferric chloride, and peracetic acid, and wherein step (b) includes the step of: causing the zeolite crystal mixture to come in contact with liquid sulfuric acid, whereby said zeolite crystal mixture is activated to produce the chlorine dioxide.
 10. A method for producing chlorine dioxide, including the steps of: (a) providing a zeolite crystal mixture, the zeolite crystal mixture comprising zeolite crystals impregnated with a proton generating species and zeolite crystals impregnated with an oxidizer; and (b) causing the zeolite crystal mixture to come in contact with sodium chlorate solution, whereby said zeolite crystal mixture is activated to produce the chlorine dioxide.
 11. The method of claim 10, wherein the zeolite crystal mixture also comprises zeolite crystals impregnated with calcium chloride, and wherein the proton generating species is sulfuric acid.
 12. A method for producing chlorine dioxide, including the steps of: (a) providing a zeolite crystal mixture, the zeolite crystal mixture comprising zeolite crystals impregnated with sodium chlorate and zeolite crystals impregnated with a proton generating species; and (b) causing the zeolite crystal mixture to come in contact with an oxidizer, whereby said zeolite crystal mixture is activated to produce the chlorine dioxide.
 13. The method of claim 12, wherein the zeolite crystal mixture also comprises zeolite crystals impregnated with calcium chloride, wherein the proton generating species is sulfuric acid, and wherein step (b) includes the step of causing the zeolite crystal mixture to come in contact with a solution of at least one of hydrogen peroxide, ferric chloride, peracetic acid, sodium metabisulfite, and sodium bisulfite.
 14. A zeolite crystal mixture capable of being activated by excess protons to produce chlorine dioxide, said mixture comprising: zeolite crystals impregnated with sodium chlorate; and zeolite crystals impregnated with an oxidizer.
 15. The mixture of claim 14, wherein said mixture also comprises zeolite crystals impregnated with at least one of a deliquescent and a water absorbing and retaining substance.
 16. The mixture of claim 14, wherein the oxidizer is at least one of hydrogen peroxide, ferric chloride, peracetic acid, sodium metabisulfite, and sodium bisulfite.
 17. The mixture of claim 14, wherein the mixture also comprises: zeolite crystals impregnated with a proton generating species.
 18. A zeolite crystal mixture capable of being activated by an oxidizer to produce chlorine dioxide, said mixture comprising: zeolite crystals impregnated with sodium chlorate; and zeolite crystals impregnated with a proton generating species.
 19. The mixture of claim 18, wherein the proton generating species is sulfuric acid, and wherein said mixture also comprises zeolite crystals impregnated with at least one of a deliquescent and a water absorbing and retaining substance. 